This invention relates generally to a method for printing a variety of colors on an absorbent substrate with a limited number of colorants. More specifically, this invention relates to a method to extend the perceived range of colors that can be printed on a substrate beyond those that are available using conventional printing methods. This method has particular utility when used with patterning systems that produce an image made up of a number of individual pixels to which individual quantities of colorants can be applied in accordance with a pre-defined patterning scheme.
To assist in the explanation of the operation and utility of this invention in its various embodiments, the following terms and definitions shall be used. As used herein, the term xe2x80x9ccolorxe2x80x9d shall encompass the concepts of hue, value or lightness, and chroma or saturation. The term xe2x80x9cperceived colorxe2x80x9d shall mean the color perceived by a human observer at a distance at which individual pixels (as that term is defined herein) are not readily discernable. The term xe2x80x9cpixelxe2x80x9d shall be defined herein as the smallest area on the substrate onto which a controlled amount of colorant can be assigned with precision. This term is distinguishable from the term xe2x80x9cpattern element.xe2x80x9d As used herein, the term xe2x80x9cpattern elementxe2x80x9d shall refer to a single pixel, but shall also refer to a group of two or more pixels that are used, as a group, to form a pattern. Pattern elements may be arranged on a substrate surface in a tiled configuration (i.e., in abutting relationship with adjacent elements, with no gaps and no overlaps between adjacent elements) to form a pattern or image. The term xe2x80x9cpixel areaxe2x80x9d shall refer to a specific area on a substrate that comprises a pixel. The term xe2x80x9ccolorantxe2x80x9d shall mean a liquid, readily flowable ink, dye, or other liquid coloring agent. This term is also intended to include a diluent that has no intrinsic color of its own. Accordingly, the term xe2x80x9ccolorxe2x80x9d as applied to a colorant used to practice this invention can include a xe2x80x9ccolorlessxe2x80x9d colorant that is used as an in situ diluent on the substrate. The term xe2x80x9cavailable colorxe2x80x9d shall mean the perceived color resulting from the application of a single colorant to a set of pixels on an uncolored (white) absorbent substrate. The term is intended to distinguish these perceived colors, which can be obtained by using readily available colorants and no blending processes, from physically or optically blended colors. The term xe2x80x9cphysically blended colorxe2x80x9d refers to a color that is the result of a physical mixing of two or more different available colorants on the substrate, resulting in the in situ formation of a colorant with a color different from the constituent colorants. A perceived color that is produced using a diluent is a physically blended color. The term xe2x80x9coptically blended colorxe2x80x9d refers to the perceived color that is generated by the juxtaposition or arrangement (including overlap) on the substrate of different colorants, none of which are individually distinguishable at a distance. The term xe2x80x9cblended colorxe2x80x9d refers to either or both of these types of colors. The term xe2x80x9ccompound pixelxe2x80x9d refers to at least two adjacent pixels, each of which are differently colored (either with an available color or with a color resulting from the physical mixing of two or more colorants applied to that pixel). The term xe2x80x9cmetapixelxe2x80x9d refers to a group of two or more adjoining or contiguous pixels, in which at least one of the pixels has been oversaturated with colorant (i.e., the quantity of colorant within the substrate area defining that pixelxe2x80x94the pixel areaxe2x80x94exceeds the absorptive capacity of the substrate within that pixel area) and at least one other pixel in the group has been undersaturated with a different colorant (i.e., the quantity of colorant within the substrate area defining that pixel area is less than the absorptive capacity of the substrate within that pixel area). As a result, colorant migration occurs within the metapixel from an oversaturated pixel area to one or more adjoining or contiguous pixel areas that were undersaturated. This pixel-to-pixel migration leads either to physical blending with, or displacement of, colorants in the adjacent pixel areas, and is a characteristic of the metapixels of this invention. Other terms shall be introduced and defined as required.
Imaging or printing systems that use the concept of pixels to place images on substrates are in common use in the printing and textile industries, and have been the subject of numerous research and development efforts. Among such systems are those capable of patterning substrates using discrete points of colorant. These points are familiar to many as the small xe2x80x9cdotsxe2x80x9d that make up the illustrations found in newspapers and magazines printed using the gravure printing process. In that process, the printed images are comprised of many small dots of colorant, each assigned to a separately defined, specific area or pixel. The larger the colorant dot assigned to that pixel, the more effectively that pixel will influence the overall perceived color of the image containing that pixel. By varying the dot size assigned to the various pixels, it is possible to reproduce colors that are not directly represented by the available colorants. For example, a green area on a substrate can be effectively reproduced using only blue and yellow dots, or an orange area using only magenta and yellow dots. Use of this technique is not limited to gravure, but is also commonly employed in lithographic and other systems using half-tone imaging.
In these cases, the different colored dots are readily seen at very close range as separate blue, yellow, or magenta dots, but at a distance are perceived as the desired color, even though that color exists nowhere on the substrate. Alternatively, the dots may overlap and visually mixxe2x80x94perhaps a blue or magenta dot is seen xe2x80x9cthroughxe2x80x9d an overlapping yellow dotxe2x80x94so that some dots actually appear on the substrate at very close range as an optically blended color. It is important to note, however, that even in these cases where the dots overlap to form an optically blended color (similar to making colors using overlapping transparent films of different colors), the individual colorants applied to the substrate remain in discrete, intact units that can be visually identified. No physical mixing or blending takes place between the contiguous or overlapping dots, and no migration or displacement of colorant takes place between adjacent pixels.
The textile industry has been using pixel-based printing techniques for a number of years for the purpose of generating multi-colored images on various absorbent textile substrates. Examples of such printing techniques used by the assignee are described in U.S. Pat. Nos. 3,942,342; 3,969,779; 4,894,169; 5,128,876; 5,136,520; 5,142,481; 5,195,043; and 5,208,592, all of which are hereby incorporated by reference as if expressly set forth herein. These latter systems use a plurality of liquid colorant applicators that selectively apply pulsed streams of colorants onto a moving absorbent substrate. The applicators are arranged along one of several linear arrays that are positioned in parallel relationship across the path of the moving substrate. A different liquid colorant is supplied to each array. As the substrate passes under a given array, the liquid colorant associated with that array is delivered to the substrate from one or more applicators in the form of a metered stream that is directed to an individual pixel on the substrate. Colors that require blending (e.g., the green or orange discussed above, assuming those colors are not among the available colorants) are reproduced by placing the appropriate colors in adjacent pixels, so as to generate an optical blend of the proper color, or by placing separately two (or more) different colorants within the same pixel, so as to generate a physical blend within the same pixel.
Each applicator in a given array is equipped to deliver no colorant, or a variable amount of the colorant associated with that array. The amount of colorant delivered to each pixel is controlled by patterning information sent by a patterning computer to each applicatorxe2x80x94carefully timed xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d instructions that effectively form a pulsed stream of colorant that delivers the appropriate quantity of colorant to the designated pixel. A limitation in the use of such xe2x80x9cduration-controlledxe2x80x9d applicators arises from the inability of the applicators in a given array to initiate and then halt the flow of colorant within an extremely short time (thereby forming an extremely short pulse of liquid colorant). While the process described above controls colorant quantity within a pixel by controlling the duration of colorant flow, and not by varying the flow rate or orifice characteristics of the individual applicators, this invention is nevertheless applicable to other application methods using liquid colorants in which flow rate, flow character, or other means to meter colorant onto the substrate surface may be controlled, so long as there exists an inability to deliver an extremely small quantity of colorant to a designated pixel on the substrate surface. For convenience, this specific limitation shall be referred to herein as the microdelivery limitation.
As a practical matter, the micro-delivery limitation makes it impossible to reproduce any of those colors requiring the blending of colorants assigned to different arrays where the proportion of colorant needed to reproduce accurately the desired color is below the minimum quantity of colorant that the applicator physically can deliver. This limitation can also result in an inability to reproduce a color that requires the application of only a single colorant, but that must be applied so sparingly that the applicator cannot meter the colorant in such a small quantity. An example is printing a very pale blue color for which the only source of blue is a colorant that has an intrinsic dark blue color.
A known method to reduce these shortcomings is through the generation on the substrate of a repeating pattern of a set of two or more pixels, each pixel being assigned different, readily reproducible color. This set of pixels is then used in the same manner as a pixel, i.e., it is duplicated and positioned on the substrate to form the desired pattern and color. Because it is used in the manner of a pixelxe2x80x94to form a visually imperceptible individual unit of color from which the overall image is constructedxe2x80x94but is comprised of two or more adjacent pixels, this set of pixels shall be referred to as a compound pixel. The use of compound pixels can be reasonably effective in expanding somewhat the range of perceived colors that can be printed using a limited number of available colorants. This concept can be implemented using any of several arrangements of individual pixels within the compound pixel, e.g., a checkerboard, dither, or some other arrangement scheme.
This method, however, is of little utility if the ability to place very small quantities of colorant within a pixel is limited. Often in such cases, attempts at constructing and arranging pixels to form blends results in images that, when viewed, may be perceived as having the desired color, but also exhibit unintended and undesirable patterning artifacts, such as stipple, spots, mottling, heather, or moire. Such artifacts are particularly objectionable when, in order to construct the appropriate compound pixel yielding the desired color, individual pixels containing contrasting colors must be used. For example, it may be necessary to use a compound pixel comprised of a magenta and a green in a small checkerboard array in order to construct a perceived color that most closely represents the desired color, rather than using two or more colors that are less contrasting. However, by using such contrasting colors, the resulting visual xe2x80x9cblendxe2x80x9d is much more likely to be burdened with undesirable patterning artifacts as a result of the pixel geometry and the mechanisms of visual perception.
The invention disclosed herein overcomes the micro-delivery limitation in a way that is not shown or suggested in the prior art, and provides for a greatly expanded palette of colors than can be produced from a relatively small number of colorants. This result is achieved by simulating, without actually delivering, the low quantity of colorant that would otherwise be necessary to achieve the desired perceived color on an absorbent substrate. In addition, this invention also can minimize the distracting pattern artifacts that are common among prior art techniques.
If the quantity of colorant applied to a pixel area exceeds the ability of the substrate to absorb it, effectively oversaturating that pixel area, some quantity of colorant will diffuse or migrate beyond the boundaries of the pixel area and occupy a portion of an adjacent pixel area that is undersaturated. This migration of colorant will cause either a displacement of the color in an adjacent pixel area or a physical blending with the color in an adjacent pixel area. This migration can occur from pixel to pixel within a group of adjoining or contiguous pixels, as well as outwardly beyond the edge of the group, thereby causing colorant displacement or blending in areas immediately adjacent to the group. Groups of adjoining or contiguous pixels containing at least one oversaturated pixel area and at least one adjoining or contiguous undersaturated pixel area, and which exhibit pixel-to-pixel colorant migration, have been previously defined as metapixels. It has been found that the use of metapixels to form printed images has the unexpected effect of providing for the simulation of colorant dots of a much smaller size (by providing pixels that appear to contain much less of a given colorant) than is possible with single or compound pixel techniques.
Because it is frequently undesirable to oversaturate large areas of the substrate with colorant, the quantity of colorant directly applied to the adjacent pixels can be adjusted to accommodate the inter-pixel colorant migration in order to maintain the desired degree of average local substrate xe2x80x9cwet outxe2x80x9d or saturation. This level is usually xe2x80x9c100%xe2x80x9d or full saturation without oversaturation, a level which generally assures full colorant penetration and maximum xe2x80x9ccover.xe2x80x9d It should also be noted that, in addition to oversaturating certain pixels with a single colorant, it is possible to achieve an oversaturated condition using partially saturating applications of two or more colorants within the same pixel. Doing so will cause an interpixel migration of a combination of colorants, and can again create color blends that are beyond existing color generating techniques. Similarly, partially saturating applications of two or more colorants can also be assigned to a pixel that remains undersaturated, as artistic considerations may require.
It is also contemplated that the physical placement or arrangement of the individual component pixelsxe2x80x94including those that are oversaturated or undersaturatedxe2x80x94within the metapixel need not be fixed, but can be varied as needed to assist in emphasizing pattern boundaries, adjusting pattern definition, or for other reasons. The skillful construction and arrangement of th e metapixelxe2x80x94including the adept choice of the initial colorants used, careful selection of the nature and degree of colorant oversaturation and migration employed, and the judicious placement and optimal systematic rearrangement of the individual pixels within the metapixelxe2x80x94can greatly expand the effective color palette possible from a given number of available colors and a limited ability to apply small quantities of colorant. This invention yields far more perceived colors that would otherwise be possible using conventional techniques. As an additional benefit, the patterning artifacts associated with conventional techniques, noted above, can be dramatically reduced or eliminated.
The above discussion is a summary of certain deficiencies in the prior art and certain advantages of the instant invention. Other advantages will be apparent to those skilled in the art from the detailed description of the invention that follows, which description includes exemplary embodiments, as well as exemplary drawings as briefly described below.